This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to utilizing a generalized helical interpolation algorithm.
In at least some xe2x80x9ccomputed tomographyxe2x80x9d (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield units,xe2x80x9d which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
In at least one known imaging system, a single scout image is generated by fixing the position of the x-ray source and translating the object in a z-axis direction. A resulting scout image, often called a scanogram, is similar to a plain radiography image. Using the scout image, an operator may identify anatomical landmarks. However, the scout image generated from a single projection angle does not provide depth information regarding the object anatomy.
A plurality of scout scans are performed to generate depth information scout images of an object. Specifically, in order to generate at least one depth information scout image, the imaging system performs each scout scan at a different projection angle, or scout angle, with respect to the scanned object, e.g., patient. For example, as the patient is translated along a z-axis at a constant speed, a plurality of scout, or projection, data is collected as the position of a gantry is adjusted along a plurality of projection angles, or scout angles.
Typically an urology x-ray examination can take up to five hours to complete. A series of x-ray films need to be taken prior to injecting contrast media into a patient and following uptake of the contrast media. The x-ray procedure is lengthy and tedious because it takes several hours for the contrast media to be eliminated from the patient""s system. In another known application, a CT scanner and an x-ray are is used to improve diagnostic accuracy and improve the examination time. The protocol requires a combined set of planar x-ray films and CT scans. A special tabletop is placed on top of the existing CT table so that a film cassette can be placed underneath, and an x-ray tube is suspended on a ceiling above a patient table. A first set of x-ray film and CT helical scans are taken, prior to the administration of contrast media or as the contrast media is injected into the patient""s abdominal area but before the contrast media is sufficiently absorbed by the patient to impact collected data. The abdominal area is compressed by an inflated balloon to keep the contrast media within the kidneys and the upper urinary tracks. After the contrast media is absorbed by the patient, i.e., uptake of contrast media, a second set of x-ray film and CT scans is taken. The compression apparatus is then removed allowing the contrast media to leave the kidneys and upper urinary tracks, and the x-ray film and CT scans are repeated approximately thirty to forty minutes after the injection of the contrast medium.
Known CT scout images, however, include dark bands near high-density structures, e.g., bones. These dark bands, or artifacts, prevent accurate assessment of the patient""s pathology when the artifacts are near contrast filled vessels. The artifacts are caused by the known enhancement algorithm utilized in typical scout processing. This known algorithm sums multiple samples along the table travel direction for noise suppression. As a result, the spatial resolution in the z-direction, e.g., the direction of table travel, is compromised. Further, a significant mismatch between an x-resolution and z-resolution results when the minimum slice thickness, e.g., size of each sample in the z-direction, is 1.25 mm. The x-ray device can be eliminated and the CT scanner can be solely utilized to generate CT scout images.
Methods and Apparatus for reducing image artifacts when reconstructing an image with a multislice computed tomographic (CT) imaging scanner are provided. In an exemplary embodiment of the method, scout images are generated by obtaining a plurality of projection views of an object, modifying the projection data utilizing a deconvolution kernel, generating a horizontal gradient and a vertical gradient based on the modified projection data, applying weights to the horizontal gradient and vertical gradient, and applying a desired level of enhancement to the weighted horizontal and vertical gradients. The above described method generates enhanced scout images without requiring modifying the CT scanner.
In one aspect, an imaging system comprises a computer, a gantry having a detector array, an x-ray source for radiating an x-ray beam toward the detector array, and the imaging system acquires a plurality of projection views of the same projection angle of the object. The imaging system modifies the projection data utilizing a deconvolution kernel, generates a horizontal gradient and a vertical gradient based on the modified projection data, applies weights to the horizontal gradient and vertical gradient. and applies a desired level of enhancement to the weighted horizontal and vertical gradients to generate a scout image.
In another aspect, a processor in the imaging system is programmed to acquire projection data for a plurality of projection views of the object. The processor modifies the projection data utilizing a deconvolution kernel, generates a horizontal gradient and a vertical gradient based on the modified projection data, applies weights to the horizontal gradient and vertical gradient, and applies a desired level of enhancement to the weighted horizontal and vertical gradients to generate scout images.
In yet another aspect, a computer-readable medium in the imaging system is provided which comprises records of projection data for a plurality of projection views that are used to reconstruct a plurality of records of scout images. To generate the plurality of records of scout images the records of projection data are modified utilizing a deconvolution kernel, records of horizontal and vertical gradients are generated based on the records of modified projection data, a plurality of rules apply relative weights to the records of horizontal and vertical gradients, and a plurality of rules apply a desired level of enhancement to the records of weighted horizontal and vertical gradients.